Gate turn-off thyristor

Abstract

A mesa-type wide-gap semiconductor gate turn-off thyristor has a low gate withstand voltage and a large leakage current. Since the ionization rate of P-type impurities greatly increases at high temperatures when compared with that at room temperature, the hole implantation amount increases and the minority carrier lifetime becomes longer. Consequently, the maximum controllable current is significantly lowered when compared with that at room temperature. To solve these problems, a p-type base layer is formed on an n-type SiC cathode emitter layer which has a cathode electrode on one surface, and a thin n-type base layer is formed on the p-type base layer. A mesa-shaped p-type anode emitter layer is formed in the central region of the n-type base layer. An n-type gate contact region is formed sufficiently apart from the junction between the p-type anode emitter layer and the n-type base layer, and an n-type low-resistance gate region is so formed in the n-type base layer that it surrounds the anode emitter layer.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to a gate turn-off thyristor that use a wide-gap semiconductor and relates, in particular, to a gate turn-off thyristor capable of interrupting a large current within a wide temperature range. [0002] As a first background art gate turn-off thyristor (hereinafter referred to as GTO) that uses silicon, there is the one disclosed in JP H06-151823 A. In the first background art GTO, a mesa-type p-type base layer is provided on an n-type base layer that has an anode electrode, and an n-type emitter layer is formed by impurity diffusion in a central region of the mesa-type p-type base layer. With this construction, a junction between the p-type base layer and the n-type emitter layer is not exposed on the mesa slope, and therefore, a GTO in which electric field concentration hardly occurs on the mesa slope can be obtained. However, since the n-type emitter layer is formed by impurity diffusion, there are many crystal defects...

AI Technical Summary

Benefits of technology

[0018] According to the present invention, in the gate turn-off thyristor (hereinafter referred to as a wide-gap GTO) of a wide-gap semiconductor that has a mesa-type emitter layer, the maximum controllable current is increased by relieving the electric field of the insulator located in the neighborhood of the end portion of the junction between an emitter layer and a base layer where a gate is provided adjacent to the emitter layer.

[0019] In order to relieve the electric field of the insulator located in the neighborhood of the end portion of the junction, a low-resistance gate region of a low resistance value is formed in the base layer. With this arrangement, a current at the turn-off time flows through the low-resistance gate region of a low resistance value, and therefore, a voltage drop is a little. If the gate current is increased at the turn-off time by raising the off-gate voltage, the electric field of the insulator is not increased so much. As another method for preventing the increase in the electric field of the insulator, there is a method for forming a field relief region in the neighborhood of the junction. Since the electric field of the insulator is relieved by the method, the off-gate voltage can be raised. Therefore, the principal current can be diverted with high efficiency. Since the off-gate voltage can be raised, a large maximum controllable current can be maintained within a wide temperature range from a low temperature of not higher than room temperature to an elevated temperature that exceeds 500° C. When the off-gate voltage is not increased so much, the long-term reliability is remarkably improved. Since the electric field of the insulator in the neighborhood of the junction can be reduced, the long-term reliability of the GTO can be maintained.

[0021] According to the present invention, by virtue of the first conductive type low-resistance gate region formed in the first conductive type base layer, an electron current flows from the first conductive type base layer through the first conductive type low-resistance gate region and the first conductive type gate contact region to the gate at the turn-off time. Since the low-resistance gate region has a low resistance value, a voltage drop in the first conductive type base layer is small even when a current due to the electron flow is large. Therefore, the off-gate voltage applied between the anode and the gate is not influenced by the voltage drop, and a large current can be flowed with high efficiency. As a result, the controllable current of the GTO can be increased.

[0024] According to the present invention, the second electrode is put in contact with only the central region of the second emitter layer and put in contact with the second emitter layer via the contact electrode formed of a material of which the contact resistance to the semiconductor layer is lower than that of the second electrode in the other region. Therefore, the contact resistance between the second electrode and the second emitter layer in the region located with interposition of the contact electrode is lower than that of the other region. With this arrangement, a current that flows from the second electrode into the second emitter layer flows more in the peripheral region located with interposition of the contact electrode than in the central region where the resistance is relatively high. In the GTO, the current control effect by virtue of the low-resistance region is great in a portion located near the low-resistance region, but the effect is reduced in the central region remote from the low-resistance region. In the present invention, the greater part of the electrification current is flowed in the peripheral region where the current control effect by virtue of the low-resistance region is high, so that the current in the central region of a low control effect is reduced. As a result, the efficiency of extracting a current from the gate at the turn-off time is increased, and therefore, the controllable current of the GTO is increased.

[0025] According to the present invention, by sufficiently separating the gate contact region of the GTO that uses a wide-gap semiconductor from the junction between the mesa-type emitter layer and the base layer, the electric field in the neighborhood of the junction or in the neighborhood of the mesa corner portion is not increased even when the off-gate voltage is high. By raising the off-gate voltage, the current flowing between the anode and the cathode can efficiently be diverted into the gate, and the controllable current of the GTO can be increased. Moreover, since a high electric field is not applied to the insulator, the leakage current is not increased, and the long-term reliability can be maintained.

[0026] By forming the low-resistance region adjacent to the gate contact region, a voltage drop caused by the current that flows through the low-resistance region at the turn-off time can be reduced. Therefore, even when the off-gate voltage is the same as the conventional one, the turn-off current can be diverted into the gate with higher accuracy than in the conventional GTO. Even if the p-type impurity ionization rate is increased than at room temperature or the carrier lifetime become longer during use at high temperature, the off-gate voltage can be raised. Furthermore, the gate current at the turn-off time can be diverted into the gate with high efficiency by the low-resistance region. Therefore, a GTO, which has a large controllable current within a wide temperature range from a low temperature of not higher than room temperature to a temperature that exceeds 500° C. and is able to maintain high reliability for a long term, can be provided.

Problems solved by technology

However, since the n-type emitter layer is formed by impurity diffusion, there are many crystal defects, and the on-resistance of the GTO is increased.

However, in the case of SiC that is the wide-gap semiconductor, the impurity thermal diffusion is very slow and therefore not appropriate for mass production.

In the case, if high-concentration impurity ions are implanted, the crystal defects increase and the resistance becomes high.

Therefore, when a large current is flowed through the GTO, a voltage drop in the region where ions have been implanted increases, and the power loss is large.

In particular, when impurity ions of a large atomic radius of a p-type impurity of aluminum or the like are implanted, crystal defects easily occur, and a high-concentration p-type region cannot be formed without a crystal defect.

Moreover, there is a problem that, if the high electric field is continuously applied for a long term, a leakage current increases to reduce the gate withstand voltage (withstand voltage between the gate G and the anode A) of the GTO element, and the long-term reliability is degraded.

Consequently, the withstand voltage between the cathode K and the gate G is lowered, and the long-term reliability is degraded.

Moreover, the on-state voltage is also raised, which causes a problem that the power loss is increased.

Moreover, since the carrier density in the p-type anode emitter layer 4 is increased at high temperature, the depletion layer does not sufficiently spread when the off-gate voltage is applied.

Furthermore, the electric field in the neighborhood of the end region T of the cathode emitter layer 24 is increased, and the electric field of the insulator 10 is increased, possibly causing dielectric breakdown.

Moreover, the leakage current is increased, and this reduces the reliability during long-term use.

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